Communication systems consisting of land mobile radio, cellular radiotelephone, personal communication system (PCS), and various other types are well known. One such multiple access wireless communication system is a direct sequence code division multiple access (DS-CDMA) cellular communication systems, such as set forth in the TIA Interim Standard (IS)-95A, Mobile Station-Base Station Compatibility Standard for Dual-Mode Spread Spectrum Cellular Systems, Telecommunications Industry Association, Washington, D. C. Jul. 1993 IS-95A!. According to these standards, coded communication signals are transmitted in common 1.25 megahertz (MHz) carriers between the base station system (BSS) and mobile communication units that are communicating in the service coverage areas of the BSS.
Another TIA interim standard, Telecommunication Industry Association Interim Standard (IS) IS-634, MSC-BS Interface, Telecommunications Industry Association, Washington, D. C., 1995, provides interface specifications for the generalized architectural framework required to support a typical multiple access wireless communication system such as a digital radio frequency (RF) radiotelephone system. The generalized architectural framework includes multiple base transceiver stations (BTS) in communication with at least one base station controller (BSC). The generalized architectural framework also includes multiple selection distribution units (SDU) and at least one mobile switching center (MSC). The typical multiple access wireless communication system communicates via the MSC to a public switched telephone network (PSTN).
BTSs communicate via an RF channel with mobile communication units operating within their respective coverage areas, via forward (BTS to mobile communication unit) and reverse links (mobile communication unit to BTS). An SDU is coupled to one or more BTSs as well as to a BSC. In general, the SDU performs frame selection on the reverse link from the BTSs, and air interface frame distribution on the forward link to the BTSs. In addition, an inter-working function (IWF) is required for conversion of data signals from an radio link protocol such as set forth in TIAIEIA IS-707, Data Services Option Standard for Wideband Spread Spectrum Systems, Washington, D. C., 1997, to a protocol suitable for processing the data signal by the wireless communication system. The data signal may be either a circuit oriented data signal or a packet oriented data signal.
In a cellular communication system, the forward and reverse communication links are established for transmitting and receiving a signal between a mobile communication unit and a source base transceiver station. The signal may be a voice signal or a data signal. As a mobile communication unit moves out of range of the source base transceiver station, the signal quality degrades until one of the pair of the communication links is ultimately broken, or the call "dropped". To avoid loss of the communication links resulting from a dropped call, the communication links are shifted from the source BTS to a target BTS, or from a source sector to a target sector within the source BTS coverage area. This process of making the shift is commonly referred to in the cellular communication area as a handoff process. A handoff may occur during a call in progress (e.g. from a traffic channel to a traffic channel), or during the initial signaling during call set-up.
Handoffs are generally classified into three types; a soft handoff, a softer handoff and a hard handoff. A soft handoff occurs when a mobile voice or data signal is transferred from a source BTS to a target BTS, the BTSs serving different cell coverage areas. The transfer occurs while the mobile communication unit is in communication with both the source and target BTSs. Similarly, a softer handoff occurs when a mobile voice or data signal is transferred from a source sector to a target sector, both sectors associated with the same base transceiver station. The transfer occurs while the mobile communication unit is in communication with both the source and target sectors. During a soft and softer handoff, the mobile communication signal is supported simultaneously by both the source and target BTSs until the communication signal transfer to the target BTS, is complete. For a voice signal during a hard handoff, the mobile communication unit is directed to re-tune to a new carrier frequency, and/or the control of resources supporting the mobile voice communication signal is transferred from a source SDU to a target SDU. For a data signal during a hard handoff in which the source SDU is communicating with an IWF, control of resources supporting the data signal may not be fully transferred from the source SDU to the target SDU.
Currently, a variety of methods have been proposed for providing hard-handoff capability for CDMA circuit oriented and packet oriented data signals. One method requires keeping data signal calls in a soft/softer handoff as long as possible until either the handoff requirement ceases or the data signal call is dropped via an RF failure. Another method utilizes an intersystem link protocol (ISLP) as set forth in TIA/EIA IS-728, Intersystem Link Protocol, Telecommunications Industry Association, Washington, D. C., July 1997. Utilization of the ISLP method allows data signals to be framed and flagged at a target SDU, and subsequently "tunneled" from the target system to the source IWF via the source system for processing, across the MSC seams if required. When using the proposed ISLP method, the source IWF and source SDU remain engaged for delivery of the circuit oriented data signal to the PSTN despite handoff of all associated RF links from the source system to the target system. Likewise, the source IWF and source SDU remain engaged for delivery of the packet oriented data signal to a public packet data network (PPDN). Additionally, the source SDU is required to perform a conversion of the data signal from the ISLP to a frame relay switched virtual circuit (FR SVC) format for transporting across a mobile data path (MDP) to the source IWF. However, radio link protocol (RLP) termination and selector functions move from the source SDU to the target SDU for both circuit oriented and packet oriented data signals, upon a hard handoff. Therefore, both the source and target SDU are required for a hard handoff using the ISLP method. Specifically, the source SDU is required to continue to perform protocol conversion for processing by the source IWF during the life of the data signal call even after a hard handoff to the target BTS.
Unfortunately, utilizing the ISLP method for providing hard handoff capability for data calls as specified in the IS-634 standard, requires considerable protocol conversion, protocol processing, and resource overhead at the source and target SDU. In addition, the current IS-634 standard is not explicit with respect to the allocation of functionality required for the source and target SDUs during a hard handoff. Also, the IS-634 standard does not define which type of data signal is to be framed and flagged when utilizing the ISLP method. Further, inter-vendor inter-operability issues are likely to arise, thus adversely affecting implementation of hard handoff capability in various multiple access wireless systems due to incompatible architecture/software utilized by the manufacturers of multiple access wireless systems.
Therefore, a need exists for a method for transferring a data signal in a wireless communication system which overcomes the problems inherent with utilizing prior art methods.